After Gove and Hammond's interventions, the Prime Minister will find it harder to sit on the fence at his press conference with Obama today.

Whether or not they would vote to leave the EU in its present form is rapidly becoming a eurosceptic virility test for Conservative cabinet ministers. After initially hesitating on The Sunday Politics, Philip Hammond followed Michael Gove last night and confirmed that he would vote "out" if a referendum was held today. He told Radio 5 Live's Pienaar’s Politics:"If the choice is between a European Union written exactly as it is today and not being a part of that then I have to say that I'm on the side of the argument that Michael Gove has put forward."

Unsurprisingly, Downing Street is said to regard Gove's intervention as "unhelpful". The Education Secretary's public confirmation of last year's Mail on Sundayreport means every cabinet minister can now expect to be asked how they'd vote - and that includes David Cameron. With impeccable timing, the Prime Minister is in Washington today to help negotiate an EU-US trade deal and is holding a press conference with Barack Obama at 4:15pm. If asked whether he would vote to leave the EU today (as he surely will be), Cameron will find himself caught between the europhile US president (who regards Britain's flirtation with withdrawal as a form of madness) and the thought of his eurosceptic backbenchers. The contorted answer he produces should be worth waiting for.

As for the rest of the cabinet, Tim Montgomerie lists Iain Duncan Smith, Theresa Villiers, Chris Grayling, Justine Greening, Oliver Letwin and Francis Maude as other "definite or probable EU Outers", all of whom, if they wish to maintain the favour of the Tory grassroots, will be tempted to say they'd vote to leave today.

Gove and Hammond's remarks also revive the question of how the Prime Minister will respond if his renegotiation strategy fails (as europhiles and eurosceptics alike predict it will). Both ministers made it clear that they would only vote to stay in if Britain's terms of membership are substantially reformed. The question that will again be put to Cameron is that which shadow foreign secretary Douglas Alexander has continually asked: what percentage of your demands do you need to secure to support a Yes vote? 30 per cent, 50 per cent, 80 per cent? The PM's response is to say that no one goes into a negotiation "hoping and expecting to fail" but the pressure will now rise on him to say what would constitute failure.

Yesterday's events are a reminder of why the referendum, if it ever comes, could lead to the biggest Conservative split since the reform of the Corn Laws. If Cameron's renegotiation attempts are seen to have failed in the eyes of eurosceptics, some ministers will want to vote to leave, while others (including, undoubtedly, Cameron), will want to vote to stay; the cabinet will be split down the middle.

It's worth recalling how the last (and only) government to hold an EU referendum - Harold Wilson's Labour administration in 1975 - dealt with a comparable problem. With europhiles like Roy Jenkins on one side and eurosceptics like Tony Benn on the other, Wilson took the unprecedented step of suspending collective cabinet responsibility in order to allow his ministers to support either side in the campaign. Seven Labour cabinet ministers - Benn, Barbara Castle, Michael Foot, William Ross, Peter Shore John Silkin, Eric Varley - went on to unsuccessfully argue for withdrawal from the EEC (the vote was 67-33 in favour of membership). If and when the referendum comes, the most elegant way for Cameron to respond to a split party may be to invoke the Wilson precedent.